Researchers in the Department of Electrical and Computer Engineering at The University of Texas at Austin (UT ECE) have demonstrated the ability to perform nanoscale chemical analysis of molecular films with unprecedented sensitivity by detecting molecular photoexpansion. PhD students Feng Lu and Mingzhou Jin led by Prof. Mikhail Belkin successfully acquired high-quality infrared spectra from as few as 300 molecules in ambient conditions and achieved better than 25 nm spatial resolution. These capabilities enable a highly-sensitive nanoscale analytical tool for chemists, biologists and materials scientists. The results were published today in Nature Photonics.
Infrared spectroscopy is one of the most popular techniques in chemical analysis. The pattern of molecular infrared absorption peaks acts as a chemical fingerprint, enabling identification. Due to light diffraction, however, traditional infrared microscopy can only achieve spatial resolution of several microns.
In 2005, French researcher Alexander Dazzi and colleagues showed that infrared absorption in polymer films can be recorded by observing polymer photoexpansion with an atomic force microscope (AFM) probe, but the sensitivity of the technique was limited to samples with over 100 nm in thickness.
By employing a combination of electromagnetic and mechanical enhancement mechanisms, researchers at The University of Texas at Austin achieved approximately two orders of magnitude improvement in sensitivity, enabling for the first time broadband high-quality nanoscale infrared spectroscopy of films as thin as a single molecule.
The University of Texas team used a sharp metal-coated AFM tip to focus infrared laser light into an intense spot localized below the nanometer-sized tip apex. The molecules under the tip absorb optical energy and undergo rapid thermal expansion which produces a momentary force on the AFM tip. The researches managed to translate this minuscule force into detectable cantilever deflection by sending light pulses at a repetition frequency that coincided with a mechanical resonance frequency of the AFM cantilever. By scanning the wavelength of the laser pulses and measuring the cantilever deflection amplitude, the team was able to record infrared absorption spectra of the molecules.
“Over the past years, the efforts to achieve high-sensitivity nanoscale infrared microscopy in ambient conditions have been focused on detecting light absorption by optical means,” says the principle investigator Mikhail Belkin, an assistant professor in the Department of Electrical and Computer Engineering. “Our approach offers a compelling alternative to these methods by identifying molecular infrared absorption spectra via recording mechanical force action of expanding molecules on the AFM tip.”
The Belkin group is now transferring this technology to a California-based company Anasys Instruments that develops a variety of tools for nanoscale materials characterization. “We are extremely excited about this research,” says Craig Prater, Chief Technology Officer at Anasys. “Extending the sensitivity of the AFM-IR technique to the scale of individual monolayers opens up a huge range of potential applications that were previously out of reach.”
The research is supported by the Robert A. Welch Foundation. The technology transfer is sponsored by the U.S. Department of Energy under the Small Business Technology Transfer program.